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6-1 Chapter 6 Roadside Maintenance and Vegetation Management for Pollinators 6.1 IntroductionÂ Roadside vegetation management provides a safe driving environment, preserves infrastructure, and maintains a resilient plant community. Many departments of transportation (DOTs) have a statewide Integrated Vegetation Management (IVM) plan or an Integrated Roadside Vegetation Management (IRVM) plan. IVM plans typically include guidance on preventing unwanted vegetation, protecting rare plants, and preserving natural areas, along with a decision-making process for managing vegetation. IVM plans can also include guidance regarding best practices to incorporate to support pollinators through vegetation management. Management of roadside vegetation can significantly influence the number and types of pollinator species that live within or use roadsides. The timing and frequency of maintenance activities are important considerations when adjusting practices to support pollinators. In particular, the life histories of imperiled pollinators, which may have very specific habitat needs or short windows for breeding activity, can be incorporated into management plans. If a roadside site has a listed species or a species that is proposed for listing, please see the information in Chapter 4 for guidance on consultation with the U.S. Fish and Wildlife Service (USFWS) regarding maintenance strategies. 6.2 ConsiderÂ PollinatorÂ LifeÂ HistoriesÂ WhenÂ TimingÂ MaintenanceÂ PracticesÂ Understanding when imperiled pollinators are present and breeding in an area can help avoid management practices that are harmful to pollinators during the life stages in which they are most vulnerable. To help guide the timing of management activities, Table 6-1 presents the flight times of adults and the larval feeding times of imperiled pollinators found in this region. Management windows are ideal times for vegetation management and are used to avoid management during times when pollinators are reproducing or after plants go to seed. There is still much to learn about the phenology of many species; there may also be variations from year to year or site to site due to weather and microclimate. As such, these management windows should be viewed as approximate recommendations. If the host plant of an imperiled PollinatorsÂ useÂ roadsideÂ habitatÂ beyondÂ theÂ recoveryÂ zone.Â PhotoÂ Credit:Â IdahoÂ TransportationÂ DepartmentÂ ImmatureÂ stagesÂ ofÂ imperiledÂ pollinators,Â suchÂ asÂ theseÂ AtalaÂ butterflyÂ (EumaeusÂ atala)Â caterpillars,Â canÂ beÂ vulnerableÂ toÂ maintenanceÂ practicesÂ suchÂ asÂ mowing.Â TimingÂ practicesÂ beforeÂ orÂ afterÂ larvaeÂ areÂ activeÂ canÂ reduceÂ mortality.Â PhotoÂ Credit:Â JudyÂ GallagherÂ
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-2 butterfly or other needed habitat is present at a roadside site within the known range of an imperiled pollinator species, there is a chance that the pollinator is also there and that management actions could result in direct mortality. It is useful, although difficult, to survey host plants for immature stages of butterflies prior to mowing or using herbicides (or proceeding with other management actions, such as prescribed grazing). This is especially helpful if management timing falls on the cusp of the recommended window for a region or if it has been an early-spring or late-fall year. The phenology of pollinator emergence and activity may also shift as climate patterns shift (Kudo and Cooper 2019). Check with local pollinator-species experts, if available, for site-specific recommendations on timing. If management must take place while immature pollinators are present: ï· Use targeted approaches to vegetation management to avoid host plants when possible and ï· Train the staff or contractors to recognize key plants and avoid mowing, spraying herbicides, or otherwise disturbing plants during the breeding season. Questions to consider when evaluating or adjusting maintenance practices for an imperiled pollinator species. 1. Does the imperiled pollinator species need particular host plants or nectar plants? If so, compile a list of needed plant species. Consult existing roadside inventories or conduct vegetation inventories of known or suspected sites for the imperiled pollinator species. 2. Where does the imperiled species overwinter? Consider potential impacts of maintenance activities on overwintering habitat. 3. Is a particular type of management more detrimental than other practices to the imperiled species? For example, is mowing at a certain time of the year more of a threat to the species than the application of a herbicide or another method of weed control? Use the maintenance method that causes the least harm to the imperiled species. 4. When is the adult active and breeding? Consider the timing of maintenance activities, based on the windows for breeding activity. 5. If it is a butterfly or moth (or another group that is active on vegetation, such as flower flies), when is the larval stage active on vegetation? Can the timing of maintenance activities be adjusted to reduce impacts on immature stages on vegetation?
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-3 Table 6-1. Periods of imperiled pollinator activity to avoid when conducting maintenance activities in the Inland Northwest. PollinatorÂ JanÂ FebÂ MarÂ AprilÂ MayÂ JuneÂ JulyÂ AugÂ SeptÂ OctÂ NovÂ Â DecÂ Morrison'sÂ bumbleÂ beeÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â (BombusÂ morrisoni)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â WesternÂ bumbleÂ beeÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â (BombusÂ occidentalis)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Suckley'sÂ cuckooÂ bumbleÂ beeÂ Â Â Â Â Â Â Â Â Â Â (BombusÂ suckleyi)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â MonarchÂ butterflyÂ Â Â Â Â (DanausÂ plexippus)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Note:Â AdultÂ flightÂ timesÂ areÂ inÂ blue,Â andÂ larvalÂ activeÂ timesÂ (forÂ speciesÂ thatÂ haveÂ larvaeÂ activeÂ outsideÂ ofÂ aÂ nest)Â areÂ inÂ orange.Â NoteÂ thatÂ dataÂ areÂ limitedÂ onÂ theÂ phenologyÂ ofÂ larvalÂ activity.Â IfÂ localÂ expertsÂ areÂ available,Â theyÂ shouldÂ beÂ consultedÂ toÂ makeÂ theÂ bestâinformedÂ decisionsÂ onÂ theÂ timingÂ ofÂ maintenanceÂ activities.Â Â
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-4 6.3 AdjustingÂ MaintenanceÂ PracticesÂ toÂ BenefitÂ ImperiledÂ PollinatorsÂ 6.3.1 Common Roadside Vegetation Management Practices Mowing How Mowing Affects Pollinators Mowing is frequently used to maintain roadside vegetation, reduce occurrences of invasive weeds and woody plants, improve driversâ sight lines, allow vehicles to pull off, and reduce the risk of wildfires. Typically, vegetation in the recovery area (also known as the clear zoneâthe band of vegetation directly adjacent to the pavement or shoulder where vehicles that have left the roadway can recover) is mowed at regular intervals to keep it short for drivers who need to regain control of their vehicles. Routine mowing within the recovery area creates an environment that is not typically used by pollinators as habitat because it does not support flowering plants or places to shelter. This section focuses on impacts and potential opportunities to support pollinators in roadside vegetation beyond the mown strip in the recovery area. Mowing can be an effective management tool for increasing or maintaining plant diversity and, likewise, pollinator habitat. In some parts of the country, early spring mowing is key to removing cool-season weedy annual grasses; fall mowing can remove litter and aid in wildflower seed dispersal. Mowing may also be used to reduce fire fuel loads in the landscape. Mowing roadside vegetation can have detrimental effects on pollinators in general but particularly imperiled pollinators. Mowing at certain times can kill pollinator eggs or the larvae present on the vegetation directly and destroy the nests of some bumble bees (Thomas 1984; Wynhoff 1998; Humbert et al. 2010; Hatfield et al. 2016; Steidle et al. 2022) and stem-nesting bees. Mowing can also affect adult pollinators indirectly by temporarily removing host plants for butterflies and moths and the plants that provide pollen and nectar (Morris 2000; Johst et al. 2006; Noordijk et al. 2009; Kayser 2014). Vegetation structure as well as other habitat features that provide overwintering sites or temporary shelter are also altered by mowing. For these reasons, mowing can cause temporary declines in the local diversity and abundance of butterflies (Munguira and Thomas 1992; Feber et al. 1996; Halbritter et al. 2015). Significant decreases in adult butterflies occur immediately following mowing; however, when not all areas are mown in the same year, populations rebound more quickly (Weber et al. 2008). In addition, higher mowing frequencies reduce native plant growth, plant diversity (Parr and Way 1988), the ability of forbs to compete with grasses (Williams et al. 2007), and the amount of nectar and pollen present on the roadside over time. More frequent mowing can also increase roadkill involving pollinators, especially butterflies (Skorka et al. 2013). Minimizing the number of times a roadside is mowed benefits many pollinator species (Halbritter et al. 2015). Moderate mowing levels (e.g., twice per season) have been shown in multiple studies to increase plant species diversity in grassland habitats (Parr and Way 1988; Forman et al. 2003; Noordijk et al. 2009). Other studies suggest that a single mowing during the growing
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-5 season (Valtonen et al. 2007) or in the fall (Entsminger et al. 2017) is more beneficial compared to two or more mowings in a year. Considering the timing of mowing and frequency is particularly important if the habitat supports imperiled pollinator species. A single ill-timed mowing event can have long-lasting consequences for species that have small and isolated populations. For example, in areas of Oregonâs Willamette Valley where the endangered Fenderâs blue butterfly (Icaricia icarioides fenderi) occurs, mowing in late spring and early summer, while caterpillars are feeding on their lupine host plants, might destroy a population. Similarly, if timed before a plant sets seed, a single mowing can affect the reproduction of desirable plants. Adjusting Mowing to Support Imperiled Pollinators The timing and frequency of mowing can have large effects on pollinators and the vegetation they rely on. Mowing practices can be modified to reduce impacts on imperiled pollinators while also maintaining roadside safety and other management objectives. Adapting maintenance practices to support imperiled pollinators requires careful consideration of the life history of the species (see Box 6-1 for an example of how to adjust practices according to the needs of a particular pollinator species). Conservation practices that benefit pollinators broadly: ï· Reduce the frequency of mowing beyond the mown strip to once or less a year. Reducing routine mowing of the entire right-of-way (fence to fence) will benefit pollinators by allowing wildflowers to bloom and thereby supply nectar and pollen as food. Mow beyond the mown strip in the recovery area only when there are well-defined objectives, such as reducing brush or maintaining lines of sight. Although most perennial wildflowers will recover from a mowing event, some may not be able to flower and reseed in the same year of a mowing event and thus will not increase their populations. Some annuals are sensitive to mowing. If mowed before they bloom, many may be unable to recover in time to bloom at a later date and may subsequently disappear from a site. Reducing mowing beyond the mown strip to once a growing season is best for wildflowers and pollinators. In some parts of this region, mowing can be reduced to every other year or even every 2 to 3 years, depending on the intervals of mowing needed to control woody plant encroachment or reinvigorate populations of wildflowers. ï· Delay mowing as late as possible during the growing season (e.g., mow after the first frost) so blooming plants are available throughout the growing season. Strategic timing of mowing can also reduce effects on pollinators and promote plant diversity. It is best to delay mowing until after the first frost, if operational constraints allow it. When mowing is delayed, butterflies and other pollinators with larval stages that reside on vegetation will be able to complete their full life cycles, and flowering plants will be able to bloom and provide pollen and nectar to pollinators uninterrupted ReducingÂ theÂ frequencyÂ ofÂ mowingÂ beyondÂ theÂ recoveryÂ zoneÂ allowsÂ wildflowersÂ toÂ flourish.Â PhotoÂ Credit:Â OregonÂ DOTÂ
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-6 throughout the growing season. Furthermore, a mowing event in the fall can help spread wildflower seeds. Fall mowing also targets woody species that can invade pollinator habitats. Litter removal (e.g., haying) can complement mowing by removing the build- up of litter that can limit wildflower germination and growth. ï· Balance vegetation management with pollinator needs. If mowing must take place during the growing season, consider selecting a time to mow that balances vegetation management needs (e.g., noxious weed control) with the resource needs of pollinators (e.g., flowers and host plants). For example, mow at a time that will promote the growth of wildflowers; though the temporary removal of flowers will be harmful in the short term, mowing will benefit pollinators in the long term. The timing of mowing can be targeted to reduce the abundance of a dominant plant species or litter and aid wildflower seed dispersal. To reduce the cover of an unwanted herbaceous species, mow when the plant is most vulnerable (i.e., during their growing season but prior to bloom or before seeds set). This action can help limit new growth of unwanted or dominant species the following year and create space for other species to grow. It is also worth considering varying the season when mowing takes place every few years to increase plant diversity. Mowing consistently at the same time every year will favor some plants over others. Plant diversity can be maintained by occasionally varying the timing of mowing, which will favor different plants and prevent certain plants from dominating the planting. ï· Clean mowing equipment after use and between sites to limit the spread of invasive weeds. Weed seeds can spread after equipment is driven through infested areas. Seeds that lodge in tires and undercarriage are jarred loose at new locations. After mowing areas with a dense infestation of weeds, spray the vehicleâs tires and undercarriage with an air compressor as soon as possible to remove weed seeds and prevent their unintentional spread. Conservation practices for imperiled pollinator species include: ï· Aim to mow no more than one-third to one-half of an area beyond the mown strip in the recovery area per year (e.g., rotate mowing sections of a roadside). Mowing only a portion of the roadside (e.g., one pass with a mower) in a given year leaves behind refugia (i.e., intact vegetation that can support pollinators). In addition, this heterogeneous mowing strategy can help prevent gaps in blooming and enhance the diversity of flowering species across the site. ï· Avoid mowing host plants of imperiled butterflies during butterfly breeding seasons. It is important to time mowing so as to avoid vulnerable stages of the life cycle of any rare or declining species that might be present, particularly during the larval (caterpillar) stage of butterflies and moths. For example, in southeastern Washington, to reduce harm to monarch butterflies, it is best to avoid mowing between June 1 and September 30 when monarchs are breeding in the region.
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-7 See Chapter 3 for natural history information for imperiled species and Table 6-1 for adult flight times and breeding seasons of imperiled species in the region. ï· Adjust mowing height to a minimum height of 8 to 10 inches in areas with target butterfly host plants or bumble bee colonies in grass thatch, if mowing during the growing season. Mowing established vegetation at a height of 8 to 10 inches (or higher, if equipment allows; see Eco 1200 from MULAG [Steidle et al. 2022]) leaves a greater depth of vegetation for pollinators to use, which is particularly important for imperiled bumble bees that build their colonies in grass thatch aboveground or caterpillars on host plants. Adjusting the mowing height also helps vegetation to recover more quickly and reduces plant stress, particularly during dry periods or drought. ï· Explore mitigation efforts if there is a known mortality hot spot for an imperiled pollinator due to vehicle collisions. Mortality hot spots can occur if landscapes funnel pollinators through high-traffic areas or the flight patterns of a particular species. Pollinators that are poor fliers or normally found flying low to the ground may be more vulnerable to vehicle collisions than pollinators that are strong fliers or normally found flying high above the ground. Mitigation efforts could include reducing traffic speeds temporarily for a mile or two near the hot spot, using fencing or netting to cause pollinators to fly higher when crossing the road, closing the outer lanes of traffic temporarily during the peak breeding season, or widening the mown strip within the recovery area to create more distance between the road and the habitat used by a pollinator (Zielin 2010; Kantola et al. 2019). Box 6-1. Adjusting Mowing for the American Bumble Bee, an Imperiled Pollinator The American bumble bee has one of the widest distributions of bumble bees in the United States, found in 46 of the lower 48 states. Photo Credit: Barbara Driscoll The American bumble bee (Bombus pensylvanicus) is found throughout the eastern and central parts of the United States as well as the Desert Southwest, California, and southern Oregon (but is absent from the Intermountain West). This species is in decline in the northern portions of its range and less common than it once was in the southern portions, with an average range decline of 51 percent. This species has a vulnerable status, meaning that the bee faces a risk of extinction, based on analyses from the International Union for Conservation of Nature. Queens emerge in late spring to search for a nest site for their colonies. This period of queen nest-seeking, as well as early foraging in spring, is a vulnerable time in the life cycle of this and other bumble bee species. The species locates an insulated aboveground cavity, usually within tall grass at the soil surface. If present at a roadside site or known to occur in the area, the American bumble bee would benefit from adjustments to mowing practices to accommodate its nesting and foraging needs. For example,
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-8 postponing mowing until after first frost in the fall would allow flowering plants to bloom and provide the bees with pollen and nectar throughout their flight season (i.e., late spring until early fall). Diverse sources of pollen are important for colony growth and production of the next generation of queen bumble bees. Mowing late in the season would prevent injury or destruction of the colony from use of the mower. If mowing must take place during the growing season, leaving a portion of the right-of-way unmown would leave some flowering resources and nesting habitat as refugia for bumble bees. Bumble bees are particularly important pollinators for wild trees, shrubs, and wildflowers. They also contribute significantly to the pollination of a number of agricultural crops. Supporting the American bumble bee through roadside management would benefit adjacent lands, in addition to conservation of the species. Box 6-2. A Snapshot of Current DOT Mowing Practices In the national survey of DOTs, respondents were asked how often their agencies engaged in different mowing practices that benefit imperiled pollinators and pollinators broadly. Many respondents reported that their agencies always or usually engaged in practices like limiting mowing frequency beyond the mown strip in the recovery area, cleaning mowing equipment between sites, and avoiding mowing host plants of pollinators of interest during the breeding season. Other practices, such as setting the mowing height to 8 to 10 inches, were less commonly used.
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-9 Box 6-3. How Do Other Wildlife Respond to Maintenance Practices Designed to Help Pollinators? Two issues often raised around reduced mowing regimes are: ï· whether reduced mowing increases collisions with deer and other large mammals and ï· how other species in need of conservation, such as grassland birds, respond to reduced mowing. One concern with mowing only once a growing season, or less, is that taller vegetation may provide cover for deer that could be too close to the highway. Clearing obstructive roadside vegetation is one strategy used to increase the ability of motorists to see large mammals near the road. A wider mown strip in the recovery area may help reduce collisions in some areas (Lavsund and Sandergren 1991). However, research to date does not indicate that reduced mowing of the right-of-way beyond the mown strip in the recovery area poses increased risks to drivers due to crashes with vehicles (Barnum and Alt 2013; Guyton et al. 2014). In fact, reduced mowing beyond the mown strip in the recovery area may decrease deer foraging in the right-of-way itself because mowing can increase the palatability of some plants (Mastro et al. 2008). The behavior of large herbivores can be modified by strategies other than mowing. For example, the roadside can be made less attractive for foraging by large mammals by avoiding known palatable species in plantings. Arizona DOT excludes plant species from their seed mixes that âgreen upâ early or are otherwise known to attract deer and other large wildlife species. They have worked with partners on some projects to plant palatable species in areas leading to designated bridged wildlife crossings or habitat adjacent to roadside rights- of-way (Brown et al. 1999). Roadside vegetation is critical habitat for birds, particularly in grassland ecosystems. Bird nesting within roadside vegetation, which increases with plant diversity, is influenced by mowing regimes (Warner 1992, 1994; Shochat et al. 2005). Mowing can kill birds that are ground nesters within roadside vegetation (e.g., waterfowl, grassland species). This encompasses all stages, from eggs to adults. Some states in grassland regions delay mowing after the breeding season (e.g., after August 1 in the Midwest) in an effort to conserve grassland birds (Kociolek et al. 2015).
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-10 Box 6-4. Communicating Mowing Guidelines to the Staff One challenge when implementing changes to maintenance practices such as mowing schedules is communicating those changes to the staff. The Ohio DOT solved this problem with use of window clings, which are placed in the cabs of mowers. The window clings have basic guidelines regarding the timing of mowing and the mowing height for different situations, along with a QR code that the staff can scan for more information. This creative solution helps minimize uncertainty among staff members when changing maintenance practices. Herbicide Application How Herbicide Applications Affect Pollinators Herbicides are used to control woody vegetation as well as target weed species and other undesirable plant species on roadsides. The control of noxious and many invasive weeds is critical in maintaining habitat for many imperiled pollinators (Schultz and Dlugosch 1999). Invasive plants outcompete host plants as well as blooming plants that provide pollen and nectar, alter the structure of habitat, and change pollinator behavior (Severns 2008). The use of herbicides can benefit pollinators by suppressing undesirable plants and encouraging the valuable native plants that provide them with food or shelter. However, used indiscriminately, herbicides can reduce the quality of roadside habitat by removing floral resources and host plants (Kremen et al. 2002a; Tscharntke et al. 2005); they may also be toxic to some pollinators (Russell and Schultz 2010; LaBar and Schultz 2012). The overuse of herbicides can weaken stands of vegetation, making them more vulnerable to weed invasions, which indirectly affect pollinators. The drifting or subsurface movement of certain products can also cause reduced floral abundance, delay the onset of flowering, and reduce pollinator visitation (Boutin et al. 2014; Bohnenblust et al. 2016). Pollinators can be sensitive to these subtle changes in the timing and quality of floral resources. Drift- level doses of herbicides can also reduce pollen germination and seed production for flowering plants, thereby reducing reproduction and replenishment of the seed bank for pollinator-supporting species (Boutin et al. 2014; Gove et al. 2007).
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-11 The indirect effects of herbicide use that reduces or eliminates host plants or flowering plants that provide pollen and nectar are more likely to have greater adverse impacts on imperiled pollinators than direct toxicity. Herbicides are designed to kill plants; they are not intended to kill insects. However, pollinators can still be exposed to herbicides through direct contact, contact with herbicide residuals, or caterpillar ingestion of treated host plants. Studies indicate that some herbicides may be toxic to butterflies, causing sublethal effects such as reduced body size or wing size, which can reduce survival (Russell and Schultz 2010; Stark et al. 2012; Bohnenblust et al. 2013; Schultz et al. 2016). Alterations in development induced by herbicides can have population-level impacts (Russell and Schultz 2010), and herbicide exposure that reduces caterpillar or pupal survivorship can reduce populations over time (Stark et al. 2012). There are unknowns about the potential toxicity of herbicides, particularly to imperiled pollinators. Few studies have measured the toxicity of herbicides for butterflies and other pollinators. How one species of butterfly or pollinator responds to an herbicide may not predict the response of another; the toxicity of herbicides to different butterfly species can vary (Brown 1987; Kutlesa and Caveney 2001; LaBar and Schultz 2012). Also, the effects of tank mixes of herbicides on pollinators are largely unknown. The effects of inert ingredients in formulated products on pollinators are also generally unknown, but there are indications that ingredients like surfactants can sometimes be more toxic to pollinators than the active ingredient alone (Mesnage and Antoniou 2018; Straw et al. 2021). What is known about how pollinators respond to contact with herbicides, including consumption, is summarized in Table 6-2. This information is based primarily on the testing of adult honey bees ahead of pesticide registration; there is very little information available on herbicide toxicity to larval bees, wild bees, or adult or larval butterflies. The timing of herbicide applications can help avoid adverse effects on vulnerable butterfly life stages significantly (e.g., developing caterpillars that consume sprayed vegetation). Table 6-2 can help determine the best time to apply herbicides to avoid negative impacts on imperiled pollinators in the region. For example, applications made during the summer or early fall would reduce contact and oral exposure for species with adults and larvae that are active earlier in the season. Restoring habitat that has been degraded by invasive species benefits imperiled butterflies. Selective herbicides can be an important tool in controlling weeds that threaten key host plants of imperiled butterflies (Schultz and Ferguson 2019). PollinatorsÂ canÂ beÂ exposedÂ directlyÂ toÂ herbicidesÂ throughÂ contactÂ withÂ herbicideÂ spraysÂ orÂ residuesÂ onÂ plants,Â or,Â inÂ theÂ caseÂ ofÂ caterpillars,Â throughÂ ingestionÂ ofÂ residuesÂ onÂ plants.Â Â PhotoÂ Credit:Â Jennifer Hopwood/Xerces SocietyÂ Â
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-12 Table 6-2. Herbicides used on roadsides and what is known of their potential direct toxicity to pollinators. HerbicideÂ RatingaÂ PrecautionsÂ forÂ UseÂ withinÂ RoadsideÂ PollinatorÂ HabitatbÂ SourceÂ 2,4âDÂ ModeratelyÂ toxicÂ toÂ honeyÂ beeÂ adultsÂ andÂ theirÂ broodÂ MayÂ beÂ repellentÂ toÂ foragingÂ honeyÂ beesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· UniversityÂ ofÂ CaliforniaÂ StatewideÂ IntegratedÂ PestÂ ManagementÂ ProgramÂ 2022Â ï· UniversityÂ ofÂ HertfordshireÂ AgricultureÂ &Â EnvironmentÂ ResearchÂ UnitÂ (AERU)Â 2022Â ï· MortonÂ andÂ MoffettÂ 1972Â ï· ElliotÂ etÂ al.Â 1979Â AminopyralidÂ Â ModeratelyÂ toxicÂ toÂ honeyÂ beesÂ uponÂ contactÂ andÂ ingestionÂ Â DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â BromacilÂ Â PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ NoÂ beeÂ precaution,Â exceptÂ whenÂ requiredÂ byÂ theÂ labelÂ orÂ regulationsÂ ï· UniversityÂ ofÂ CaliforniaÂ StatewideÂ IntegratedÂ PestÂ ManagementÂ ProgramÂ 2022Â BromoxynilÂ Â PracticallyÂ nonâtoxicÂ uponÂ contactÂ toÂ honeyÂ beesÂ orÂ bumbleÂ beesÂ ModeratelyÂ toxicÂ toÂ honeyÂ beeÂ adultsÂ uponÂ ingestionÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· UniversityÂ ofÂ CaliforniaÂ StatewideÂ IntegratedÂ PestÂ ManagementÂ ProgramÂ 2022Â ï· AERUÂ 2022Â CarfentrazoneâethylÂ ModeratelyÂ toxicÂ toÂ adultÂ honeyÂ beesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â ChlorsulfuronÂ Â PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ NoÂ beeÂ precaution,Â exceptÂ whenÂ requiredÂ byÂ theÂ labelÂ orÂ regulationsÂ ï· UniversityÂ ofÂ CaliforniaÂ StatewideÂ IntegratedÂ PestÂ ManagementÂ ProgramÂ 2022Â ClopyralidÂ PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ uponÂ ingestionÂ ModeratelyÂ toxicÂ toÂ adultÂ honeyÂ beesÂ uponÂ contactÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· EuropeanÂ FoodÂ SafetyÂ AuthorityÂ (EFSA)Â 2005Â ï· AERUÂ 2022Â
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-13 HerbicideÂ RatingaÂ PrecautionsÂ forÂ UseÂ withinÂ RoadsideÂ PollinatorÂ HabitatbÂ SourceÂ DicambaÂ Â ModeratelyÂ toxicÂ toÂ adultÂ honeyÂ beesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â DichlobenilÂ Â ModeratelyÂ toxicÂ toÂ adultÂ honeyÂ beesÂ Â DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â DiflufenzopyrÂ ModeratelyÂ toxicÂ toÂ adultÂ honeyÂ beesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â DiquatÂ dibromideÂ ModeratelyÂ toxicÂ toÂ adultÂ honeyÂ beesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â DithiopyrÂ ModeratelyÂ toxicÂ toÂ adultÂ honeyÂ beesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â DiuronÂ ModeratelyÂ toxicÂ toÂ adultÂ honeyÂ beesÂ uponÂ ingestionÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ Â flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â FlumioxazinÂ Â PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ NoÂ beeÂ precaution,Â exceptÂ whenÂ requiredÂ byÂ theÂ labelÂ orÂ regulationsÂ ï· UniversityÂ ofÂ CaliforniaÂ StatewideÂ IntegratedÂ PestÂ ManagementÂ ProgramÂ 2022Â ï· AERUÂ 2022Â FluazifopâpâbutylÂ Â PracticallyÂ nonâtoxicÂ toÂ adultÂ honeyÂ beesÂ Â ModeratelyÂ toxicÂ toÂ larvalÂ honeyÂ beesÂ ToxicÂ toÂ butterfliesÂ exposedÂ asÂ larvae,Â includingÂ someÂ imperiledÂ speciesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â MakeÂ applicationsÂ outsideÂ seasonÂ whenÂ caterpillarsÂ areÂ activeÂ (seeÂ ChapterÂ 3Â forÂ adultÂ andÂ larvalÂ activeÂ periodsÂ byÂ species)Â ï· UniversityÂ ofÂ CaliforniaÂ StatewideÂ IntegratedÂ PestÂ ManagementÂ ProgramÂ 2022Â ï· U.S.Â EnvironmentalÂ ProtectionÂ AgencyÂ (EPA)Â 2019Â ï· RussellÂ andÂ SchultzÂ 2010Â FluroxypyrÂ ModeratelyÂ toxicÂ toÂ adultÂ honeyÂ beesÂ uponÂ ingestionÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-14 HerbicideÂ RatingaÂ PrecautionsÂ forÂ UseÂ withinÂ RoadsideÂ PollinatorÂ HabitatbÂ SourceÂ FosamineÂ PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ NoÂ beeÂ precaution,Â exceptÂ whenÂ requiredÂ byÂ theÂ labelÂ orÂ regulationsÂ ï· AERUÂ 2022 GlyphosateÂ ModeratelyÂ toxicÂ toÂ honeyÂ beeÂ adultsÂ andÂ theirÂ brood,Â withÂ impactsÂ onÂ gutÂ biota,Â navigation,Â andÂ survivalÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· UniversityÂ ofÂ CaliforniaÂ Statewide IntegratedÂ PestÂ ManagementÂ Program 2022 ï· MottaÂ etÂ al.Â 2018,Â 2020 ï· BattistiÂ etÂ al.Â 2021 HalosulfuronâmethylÂ PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ NoÂ beeÂ precaution,Â exceptÂ whenÂ requiredÂ byÂ theÂ labelÂ orÂ regulationsÂ ï· AERUÂ 2022 ImazapicÂ PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ NoÂ beeÂ precaution,Â exceptÂ whenÂ requiredÂ byÂ theÂ labelÂ orÂ regulationsÂ ï· AERUÂ 2022 ImazapyrÂ Â ModeratelyÂ toxicÂ toÂ honeyÂ beesÂ uponÂ ingestionÂ Â PracticallyÂ nonâtoxicÂ uponÂ contactÂ ToxicÂ toÂ butterfliesÂ exposedÂ asÂ larvae,Â includingÂ someÂ imperiledÂ speciesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â Â MakeÂ applicationsÂ outsideÂ seasonÂ whenÂ caterpillarsÂ areÂ activeÂ (seeÂ ChapterÂ 3Â forÂ adultÂ andÂ larvalÂ activeÂ periodsÂ byÂ species)Â ï· AERUÂ 2022 ï· StarkÂ etÂ al.Â 2012 ï· WashingtonÂ StateÂ DepartmentÂ of AgricultureÂ 2009Â IndaziflamÂ PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ NoÂ beeÂ precaution,Â exceptÂ whenÂ requiredÂ byÂ theÂ labelÂ orÂ regulationsÂ ï· AERUÂ 2022 IsoxabenÂ PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ NoÂ beeÂ precaution,Â exceptÂ whenÂ requiredÂ byÂ theÂ labelÂ orÂ regulationsÂ ï· UniversityÂ ofÂ CaliforniaÂ Statewide IntegratedÂ PestÂ ManagementÂ Program 2022 ï· AERUÂ 2022 MecopropâpÂ ModeratelyÂ toxicÂ toÂ honeyÂ beesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-15 HerbicideÂ RatingaÂ PrecautionsÂ forÂ UseÂ withinÂ RoadsideÂ PollinatorÂ HabitatbÂ SourceÂ MetsulfuronâMethylÂ Â ModeratelyÂ toxicÂ toÂ honeyÂ beesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â NorflurazonÂ Â PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ NoÂ beeÂ precaution,Â exceptÂ whenÂ requiredÂ byÂ theÂ labelÂ orÂ regulationsÂ ï· UniversityÂ ofÂ CaliforniaÂ StatewideÂ IntegratedÂ PestÂ ManagementÂ ProgramÂ 2022Â ï· AERUÂ 2022Â OryzalinÂ Â ModeratelyÂ toxicÂ toÂ honeyÂ beesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â OxadiazonÂ Â PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ withÂ acuteÂ exposureÂ ToxicÂ uponÂ chronicÂ exposureÂ toÂ honeyÂ beeÂ adultsÂ andÂ larvaeÂ AvoidÂ regular/repeatedÂ useÂ onÂ floweringÂ plantsÂ withinÂ theÂ growingÂ seasonÂ toÂ avoidÂ chronicÂ exposureÂ ï· EPAÂ 2020Â OxyfluorfenÂ PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ NoÂ beeÂ precaution,Â exceptÂ whenÂ requiredÂ byÂ theÂ labelÂ orÂ regulationsÂ ï· AERUÂ 2022Â Â PendimethalinÂ PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ NoÂ beeÂ precaution,Â exceptÂ whenÂ requiredÂ byÂ theÂ labelÂ orÂ regulationsÂ ï· UniversityÂ ofÂ CaliforniaÂ StatewideÂ IntegratedÂ PestÂ ManagementÂ ProgramÂ 2022Â PenoxsulamÂ ModeratelyÂ toxicÂ toÂ honeyÂ beesÂ andÂ bumbleÂ beesÂ uponÂ ingestionÂ Â PracticallyÂ nonâtoxicÂ uponÂ contactÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â Â ï· AERUÂ 2022Â PicloramÂ Â ModeratelyÂ toxicÂ toÂ honeyÂ beesÂ uponÂ contactÂ orÂ ingestionÂ Â RepellentÂ toÂ foragingÂ honeyÂ beesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â ï· EFSAÂ 2009Â ï· ElliotÂ etÂ al.Â 1979Â Â
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-16 HerbicideÂ RatingaÂ PrecautionsÂ forÂ UseÂ withinÂ RoadsideÂ PollinatorÂ HabitatbÂ SourceÂ PyraflufenÂ Â PracticallyÂ nonâtoxicÂ toÂ moderatelyÂ toxicÂ toÂ honeyÂ beesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· UniversityÂ ofÂ CaliforniaÂ StatewideÂ IntegratedÂ PestÂ ManagementÂ ProgramÂ 2022Â ï· EFSAÂ 2015Â RimsulfuronÂ ModeratelyÂ toxicÂ toÂ honeyÂ beesÂ uponÂ ingestionÂ Â PracticallyÂ nonâtoxicÂ uponÂ contactÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â SethoxydimÂ ModeratelyÂ toxicÂ toÂ honeyÂ beesÂ ToxicÂ toÂ butterfliesÂ exposedÂ asÂ larvae,Â includingÂ someÂ imperiledÂ speciesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â MakeÂ applicationsÂ outsideÂ seasonÂ whenÂ caterpillarsÂ areÂ activeÂ (seeÂ ChapterÂ 3Â forÂ adultÂ andÂ larvalÂ activeÂ periodsÂ byÂ species)Â ï· UniversityÂ ofÂ CaliforniaÂ StatewideÂ IntegratedÂ PestÂ ManagementÂ ProgramÂ 2022Â ï· AERUÂ 2022Â ï· RussellÂ andÂ SchultzÂ 2010Â ï· StarkÂ etÂ al.Â 2012Â ï· SchultzÂ etÂ al.Â 2016Â SâmetolachlorÂ ModeratelyÂ toxicÂ toÂ honeyÂ beesÂ uponÂ ingestionÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â SulfentrazoneÂ Â ModeratelyÂ toxicÂ toÂ honeyÂ beesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â SulfometuronâMethylÂ Â PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ NoÂ beeÂ precaution,Â exceptÂ whenÂ requiredÂ byÂ theÂ labelÂ orÂ regulationsÂ ï· UniversityÂ ofÂ CaliforniaÂ StatewideÂ IntegratedÂ PestÂ ManagementÂ ProgramÂ 2022Â SulfosulfuronÂ ModeratelyÂ toxicÂ toÂ honeyÂ beesÂ DoÂ notÂ applyÂ toÂ orÂ allowÂ toÂ driftÂ ontoÂ flowersÂ inÂ bloomÂ (includingÂ weeds)Â ï· AERUÂ 2022Â TebuthiuronÂ Â PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ NoÂ beeÂ precaution,Â exceptÂ whenÂ requiredÂ byÂ theÂ labelÂ orÂ regulationsÂ ï· U.S.Â DepartmentÂ ofÂ Agriculture,Â U.S.Â ForestÂ ServiceÂ 2016Â
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-17 HerbicideÂ RatingaÂ PrecautionsÂ forÂ UseÂ withinÂ RoadsideÂ PollinatorÂ HabitatbÂ SourceÂ TriclopyrÂ Â PracticallyÂ nonâtoxicÂ toÂ honeyÂ beesÂ ToxicÂ toÂ butterfliesÂ exposedÂ asÂ larvae,Â includingÂ someÂ imperiledÂ speciesÂ AvoidÂ regular/repeatedÂ useÂ onÂ floweringÂ plantsÂ MakeÂ applicationsÂ outsideÂ seasonÂ whenÂ caterpillarsÂ areÂ activeÂ (seeÂ ChapterÂ 3Â forÂ adultÂ andÂ larvalÂ activeÂ periodsÂ byÂ species)Â ï· EPAÂ 2017Â ï· StarkÂ etÂ al.Â 2012Â a.Â TheÂ dataÂ areÂ incompleteÂ becauseÂ ofÂ limitationsÂ onÂ herbicideÂ testingÂ onÂ terrestrialÂ insects.Â ToxicityÂ ratingsÂ areÂ basedÂ primarilyÂ onÂ acuteÂ contactÂ toxicityÂ testingÂ conductedÂ onÂ adultÂ honeyÂ beesÂ asÂ partÂ ofÂ theÂ EPAÂ pesticideÂ registrationÂ process.Â AdultÂ honeyÂ beesÂ (ApisÂ mellifera)Â areÂ usedÂ asÂ aÂ surrogateÂ forÂ otherÂ terrestrialÂ insectsÂ forÂ theÂ purposesÂ ofÂ EPAÂ ecologicalÂ riskÂ assessment,Â butÂ toxicityÂ ratingsÂ reportedÂ forÂ adultÂ honeyÂ beesÂ mayÂ notÂ adequatelyÂ representÂ toxicityÂ toÂ larvalÂ honeyÂ bees,Â otherÂ speciesÂ ofÂ bees,Â adultÂ orÂ larvalÂ butterflies,Â orÂ otherÂ pollinators.Â TheseÂ ratingsÂ generallyÂ doÂ notÂ accountÂ forÂ oralÂ toxicityÂ (i.e.,Â whenÂ anÂ herbicideÂ isÂ eaten,Â asÂ opposedÂ toÂ contactÂ onÂ theÂ outsideÂ ofÂ theÂ body),Â chronicÂ toxicity,Â orÂ otherÂ endpointsÂ besidesÂ adultÂ beeÂ mortality,Â exceptÂ whereÂ notedÂ withÂ additionalÂ references.Â b.Â TheseÂ areÂ notÂ theÂ pollinatorÂ precautionaryÂ statementsÂ foundÂ onÂ pesticideÂ labels.Â TheseÂ precautionsÂ goÂ beyondÂ theÂ label.Â AlwaysÂ readÂ theÂ labelÂ andÂ knowÂ andÂ followÂ theÂ applicableÂ lawsÂ andÂ regulationsÂ beforeÂ makingÂ anyÂ pesticideÂ application.Â Â
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-18 Adjusting Herbicides to Support Imperiled Pollinators Best herbicide practices for imperiled pollinators include reducing herbicide exposure as much as possible when working in their habitat. Employ a variety of vegetation management techniques, including cultural and mechanical control. When herbicides are used, avoid direct application to host plants or plants in bloom; make applications when caterpillars are not present whenever possible (see Table 6-1 for caterpillar activity windows). As with mowing, consideration of the life history of target imperiled pollinator species will help determine the conservation practices that are most relevant. By using herbicides as efficiently as possible, the maintenance staff can reduce both the amount applied and the effect on plants that benefit pollinators. Using products selectively, timing applications carefully, and following label directions can increase the effectiveness of herbicide use and decrease impacts on pollinators and other resources. Box 6-5. Weeds and Pollinators Some noxious and invasive problematic weeds can provide resources for pollinators (Harmon-Threatt and Kremen 2015). However, nonnative plants typically support only a subset of the overall pollinator community (Tallamy and Shropshire 2009). In addition, noxious weeds reduce overall plant diversity, which also reduces pollinator diversity (Memmott and Wasser 2002; Zuefle et al. 2008). When noxious and invasive weeds are removed and plant diversity recovers, pollinator abundance and diversity rebounds as well (Hanula and Horn 2011; Fiedler et al. 2012). Therefore, although monarch butterflies may be seen nectaring on Canada thistle (Cirsium arvense), the value gained from keeping those thistles is much smaller than the risks posed by retaining the noxious or invasive weeds in the landscape. If the plants that pollinators are visiting are high priorities for weed removal, use by pollinators should not preclude removal. If controlling a dense weed infestation, reseed the area after treatment with native flowering plants. Conservation practices that benefit pollinators broadly: ï· Train the staff and contractors to recognize native plants as well as noxious and invasive weeds to reduce unintended damage to nontarget plants. Conducting training in weed identification, as well as using plant identification reference materials to recognize noxious and invasive weeds, will help distinguish such species from similar nontarget species. ï· Use targeted herbicide applications on undesirable plants to avoid harming nontarget species (e.g., spot treatment applications with a backpack sprayer, targeted applications to cut stems, etc.). To avoid weakening or affecting flowering and seed set among desirable species, weeds can be targeted using spot treatment applications made with a backpack sprayer, weed wiper, or similarly appropriate technology. Using highly targeted applications on cut stems, on stumps, or under bark can also ThisÂ sprayer,Â connectedÂ toÂ anÂ ATV,Â allowsÂ forÂ herbicideÂ spraysÂ directedÂ toÂ focalÂ weeds.Â PhotoÂ Credit:Â Steve ManningÂ
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-19 reduce unnecessary effects on desirable plants. Use broadcast treatments or pellet dispersal only for dense infestations of weeds, the safety zone, and guardrail treatments. ï· Apply herbicides during plant life stages when weeds are most vulnerable (e.g., before blooming or before going to seed). Applications can be timed to be most effective, based on the herbicideâs mode of action and the application technique. For example, when using a systemic herbicide (i.e., absorbed by the plant and transported throughout the plant by the vascular system), perennial weeds can be treated in late summer and fall. During this period, perennials begin to move sugars down to their roots; the herbicide is translocated to vegetative reproductive structures where it is most effective at controlling the plant. Applications of herbicides when the weed is most vulnerable are the most successful applications. For many weeds, this is the seedling or rosette stage. ï· Use selective herbicides whenever possible to reduce damage to nontarget plants. It is important to use products and application rates that are effective in controlling target weed species, with minimal effects on nontarget species. Whenever possible, the use of selective herbicidesâthose formulated to control specific weeds or groups of weedsâcan reduce damage to nontarget plants. Nonselective herbicidesâthose that are broad-spectrum products and kill or damage all plantsâcan also be used selectively to reduce effects on nontarget plants. For example, nonselective herbicides can be used selectively by applying them on weeds when desirable native plants are dormant and using directed or targeted applications (e.g., spot spraying). In addition, understanding how quickly the herbicide being applied degrades on the site ensures that seed germination and restoration planting are not negatively affected. Herbicide labels include information about selectivity and persistence. ï· Avoid herbicide sprays when weather conditions increase drift (e.g., avoid wind speeds above 15 mph, and avoid applications during a temperature inversion). It is best to avoid applications when wind speeds are greater than 15 mph and during a temperature inversion (i.e., when warmer air above traps cooler air near the ground). During inversions, herbicides and other pesticides can linger in the air and travel long distances off-site with air movement. Reducing the off-site movement of herbicides and using nonselective broadcast applications can help avoid damage to the nontarget plants that provide pollinators with food or shelter. ï· Choose and calibrate equipment with drift management in mind (e.g., calibrate equipment regularly, choose spray nozzles that reduce drift, and on boom sprayers, use the lowest effective pressure and largest droplet size possible). The off-site movement of herbicides can be reduced by selecting appropriate spray equipment, periodically calibrating equipment, and adhering to instructions on the pesticide label. Nozzles that produce larger droplets are less likely to cause herbicides to drift off target. Equipment that is calibrated regularly limits over and under applications. ï· Avoid broadcast applications of systemic herbicides and persistent herbicides with long residual periods to reduce the exposure to butterfly and moth caterpillars, which can be exposed by consuming contaminated vegetation.
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-20 Butterfly and moth caterpillars consume host plant vegetation over days to weeks; residues in host plant vegetation could account for the bulk of exposure over the life cycle of these species. Larval butterflies and moths could be chronically exposed to systemic herbicides that are taken up by host plants or the residues of persistent herbicides, which take weeks to dissipate in the environment and have detrimental sublethal effects (Olaya-Arenas et al. 2020). See Table 6-2 for information on herbicide toxicity to pollinators. ï· Whenever possible, prevent conditions that would allow incompatible vegetation or noxious and invasive species to establish or reestablish. Control woody plants that resprout or sucker to stop regrowth and encourage desirable early successional vegetation. Monitor vegetation regularly to stay on top of emerging non-compatible vegetation issues. Track weed outbreaks and herbicide usage over time to evaluate the effectiveness of herbicide treatments and inform future management decisions. ï· If necessary (e.g., if the seed bank is depleted of desirable species), replant areas that have been treated with herbicides to remove dense infestations of undesirable vegetation with desirable, competitive low-growing plant species to reduce the need to re-treat the area. Revegetation with appropriate native plants can support pollinators while also helping to reduce weed pressure and need. Revegetation will also provide food for pollinators that may have been using noxious weeds or other undesirable plants (see Chapter 7 for more). ï· Avoid the use of pre-emergent herbicides on areas where the seed bank might contain desirable native species. Native species can be beneficial to pollinators and other wildlife; therefore, choose other methods of weed suppression in areas with desirable species. Conservation practices for imperiled pollinator species include: ï· Train the staff and contractors to recognize and avoid herbicide applications to key host plants for target imperiled butterflies and moths or key nectar plants for bumble bees. Recognition of the plants that are important to imperiled pollinators by the maintenance staff is a strategy that can help reduce applications to nontarget plants. ï· Avoid use of products that have toxicity to imperiled species during breeding seasons. Time the use of fluazifop-p-butyl and sethoxydim, which most likely have sublethal effects on imperiled butterflies (e.g., Taylorâs checkerspot [Euphydryas editha taylori], Mardon skipper [Polites mardon], Puget blue [Plebejus icarioides blackmorei]), after caterpillar activity (e.g., make applications in late summer or early fall). See Table 6-1 for times when adults and larvae are active in PlantÂ identificationÂ trainingÂ canÂ improveÂ roadsideÂ managementÂ andÂ canÂ beÂ aÂ valuableÂ professionalÂ developmentÂ skillÂ forÂ staffÂ toÂ acquire.Â PhotoÂ Credit:Â Iowa Living Roadway Trust FundÂ
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-21 this region and Table 6-2 for a list of herbicides with high toxicity to pollinators. ï· If treatment cannot be scheduled outside the window when pollinators are present, consider a mechanical control strategy when feasible if in an area where herbicide use might affect imperiled species. Avoid direct applications to or near host plants of imperiled species or sources of pollen and nectar. Box 6-6. Current Herbicide Practices Used by DOTs Many of the practices to reduce herbicide risk to pollinators were commonly used by DOTs across the country. More than half of the respondents reported using selective herbicides, applying herbicide when weeds are most vulnerable, and using targeted applications. Practices that involved staff training or monitoring and tracking roadside vegetation were less common.
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-22 BoxÂ 6â7.Â AdjustingÂ HerbicideÂ PracticesÂ toÂ SupportÂ MonarchÂ ButterfliesÂ One of the most iconic pollinator species, the monarch butterfly (Danaus plexippus), is recognized and celebrated by people throughout North America. The butterflyâs annual migration stretches from southern Canada to Mexico, covering most of the lower 48 states of the United States during the spring and summer. Threatened by habitat loss, climate change, pesticides, and other stressors, the species is now a candidate for listing under the Endangered Species Act. Monarchs rely on milkweeds (Asclepias spp.) as host plants and require nectar sources during their spring migration, breeding season, and fall migration. A diversity of milkweed species is found on roadsides. Monarchs readily lay their eggs on roadsides and consume nectar from milkweed plants. Herbicide practices can be tweaked to reduce the impacts of herbicides on monarchs. Using targeted application methods to avoid treatments to nontarget plants is important to protect milkweeds and nectar plants. When available, use selective herbicides that are targeted to the plants in need of control. For example, when appropriate, the use of tree growth regulators is preferred for monarchs rather than broadcast use of a broadleaf herbicide, which would remove nectar and host plants. If possible, avoid broadcast applications during the monarch breeding and migration season to reduce herbicide exposure to monarchs (e.g., make applications in early spring or late fall) (see Chapter 3 for a map of management timing windows across the nation for this species). Mechanical Weed and Brush Removal Weed populations can be opportunistically removed mechanically during road construction. Removal can be accomplished by excavating soils within the populations at least a foot deep and transporting the weed-contaminated material off-site for disposal (e.g., at a dump or similar approved site) or burial at the depths recommended in each state. Brush may also be mechanically removed from roadsides. Mechanical trimming to remove problematic shrubs or trees, as well as selective trimming to partially remove woody vegetation (e.g., opening up the canopy along forest edges), can benefit pollinators by creating opportunities for wildflowers to grow. However, complete removal of trees and shrubs is not always beneficial because many butterflies and moths use woody native plants as hosts or for roosting during the flight period; some tunnel-nesting bees use the stems of some shrubs or old beetle borer tunnels in snags as nesting sites. The removal of brush or trees that could be hazardous to motorists is important. This includes trees that are dangerous fixed objects as well as those that could impede sight distance, fall onto the highway, or shade the road in winter, thereby creating patches of ice. When possible, consider leaving snags or trees with cavities in areas where they are set back from the road and pose no safety risk. Snags can provide nesting habitat for some bees as well as habitat for birds and bats. Transitional areas between forest and grass can be created by using brush removal to feather or soften forest edges adjacent to the recovery area. Edge feathering involves thinning portions of the forest canopy along the edge next to grassy areas and removing undesirable or unhealthy trees. Periodic cutting to maintain healthy growth and an open canopy benefits remnant patches of savanna, forest, or other habitat dominated by woody vegetation; improves the quality of the habitat for pollinators and many birds; and is aesthetically pleasing.
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-23 Conservation actions that support pollinators generally include: ï· Feather or soften forest edges adjacent to the recovery area to create a transitional area between forests and grass (e.g., thin portions of the forest canopy along the edge next to grassy areas, removing undesirable or unhealthy trees). Shrubs and small trees that grow on the edges of forests provide food, nesting habitat, and overwintering habitat for pollinators. Many flowering shrubs and trees bloom at a time of year when other resources are scarce. They can be particularly important to early emerging pollinators like queen bumble bees. ï· Leave snags or trees with cavities in areas where they are set back from the road and pose no safety risk. Snags that are not a hazard can be home to tunnel- and cavity- nesting pollinators like yellow-faced bees (Hylaeus spp.). ï· Minimize soil disturbance (e.g., disking, tilling, removal of root balls) during brush removal activities to avoid spreading invasive plants and destroying overwintering sites or nests. Soil disturbances can bring new weed seeds to the surface, and deeper soil disturbances can destroy bee nests underground, as well as overwintering habitat for butterflies, moths, flies, and beetles. Conservation actions that support imperiled pollinators include: ï· Time activities to avoid vulnerable times of pollinator life stages. If taking action to support an imperiled butterfly or moth, avoid peak breeding periods to prevent trampling. See Table 6-1 for times when butterfly adults and larvae are active in this region. If taking action to avoid harming bumble bees, time activities during summer or early fall, outside the nest initiation phase that occurs in the spring and the overwintering phase that occurs during the dormant season when queens are most vulnerable. SofteningÂ theÂ edgesÂ ofÂ forestsÂ byÂ cuttingÂ backÂ treesÂ andÂ allowingÂ shrubsÂ toÂ flourishÂ canÂ makeÂ anÂ aestheticallyÂ attractiveÂ borderÂ andÂ provideÂ additionalÂ habitatÂ forÂ pollinators.Â Â PhotoÂ Credit:Â JeffÂ NorciniÂ
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-24 Box 6-8. Current Brush Removal Practices by DOTs Most respondents reported using practices that minimize soil disturbance during brush removal. Leaving snags, where possible, was slightly less common; creating transitions between forested areas and roadside vegetation was not common. Â Manual Weed Removal Hand pulling is the most exhaustive and complete method of controlling specific weeds because it is highly targeted. Hand weeding is well suited for removing weeds that occur in low numbers or are scattered throughout a site. Hand weeding is often the least intrusive method of removing weedy species, thereby having the least effect on pollinators. Tools that help with hand weeding include hoes, picks or Pulaski axes, trowels, and shovels. It is important to target weeds during their active growth stages, before weeds have flowered and set seed. For perennial and rhizomatous species, as much of the root material as possible is removed because plants can resprout from root fragments. One strategy for hand pulling is the Bradley Method (Fuller and Barbe 1985), which prioritizes the areas to be weeded, beginning in undisturbed areas first, then working out toward more heavily infested areas. When pulling weeds by hand or with tools, it is useful to minimize soil disturbance so that new weeds will not become established. It is also important to dispose of weeds in designated areas. If weed removal results in large patches of bare soil, recolonization by unwanted species can be reduced by seeding with desirable species. 6.3.2 Less Common Roadside Vegetation Management Practices Haying In some states, landowners are permitted to cut and remove roadside vegetation for animal fodder. States might grant emergency hay permits under drought conditions, for example,
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-25 or allow annual haying by adjacent landowners on certain roads throughout the growing season. Haying is not a tool that is typically used by a roadside maintenance staff, although there may be advantages to haying for vegetation management for DOTs to consider for future usage. For example, the removal of hayed vegetation can be beneficial for plant diversity by reducing the amount of plant litter (also known as thatch or duff), which can suppress new seedlings or existing growth. The removal of vegetation through haying also removes nutrients and creates low-fertility conditions that encourage a greater diversity of plants by favoring wildflowers rather than grasses or invasive species (Foster et al. 2009). In general, haying once in the middle of the growing season can favor wildflowers and cool- season grasses that are often suppressed by dominant warm-season grasses. However, too- frequent haying in a growing season can reduce vegetation abundance and diversity over time (Jacobsen et al. 1990), thereby reducing floral resources for pollinators. If an entire site is hayed at one time, the abrupt removal of flowers and host plants from a site can have severe consequences for rare or endemic pollinator species, depending on the timing. When possible, the maintenance staff that conducts haying or communicates with landowners could consider haying only a portion of a site at one time, thereby leaving a refuge for pollinators. In addition, setting mower blades at 10 inches or higher, when equipment allows, reduces the effect on vegetation structure that provides nesting and overwintering habitat and allows vegetation to recover and bloom more quickly. Classical Biological Control Classical biological control is the process of introducing natural enemies of a target species to limit its growth and spread. The natural enemies are insect herbivores, nematodes, or pathogens that suppress the plant within its native range. Because classical biological control involves introducing these enemies into regions outside of their (and the weedâs) native range, introductions are highly regulated by the U.S. Department of Agriculture and monitored by government scientists, university researchers, and state agencies. Although biological weed control is not currently widely implemented by state departments of transportation, some have worked with partners to release natural enemies to weeds such as purple loosestrife (Lythrum salicaria), leafy spurge (Euphorbia esula) (Johnson 2000), yellow starthistle (Centaurea solstitialis), and Russian thistle (Salsola kali) (Harper-Lore et al. 2013). Biological control can be an effective and focused approach to weed control. However, there are ecological and economic risks associated with introducing a species outside its natural range, including unpredictable and irreversible consequences (Simberloff and Stiling 1996). The Eurasian weevil (Rhinocyllus conicus), for example, was introduced to control musk thistle (Carduus nutans) but expanded its host plants to include native thistles after introduction, including several rare thistle species (Louda et al. 1997). The loss of native thistles or other native species affects wildlife species that depend on the plants, including pollinators that visit the plants for pollen and nectar or use the plants as hosts for their caterpillars. It is best to avoid using natural enemies that have expanded their hosts to include native plants. This could reduce food sources and alter habitat quality for imperiled pollinators. Coordinating carefully with state agencies and researchers, as well as keeping records about the locations of releases and monitoring the target weed populations and potential
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-26 nontarget native species, is important in evaluating the potential effects of biocontrol agents. Prescribed Grazing Grazing is used to limit tree and shrub invasion, provide structural diversity, and encourage the growth of nectar-rich plants. However, livestock grazing is beneficial to plant diversity, and, in turn, pollinators, at only low to moderate levels during short periods, which are separated by long periods of recovery (Hopwood et al. 2016a). Grazing can negatively affect insect communities by changing plant community structure and diversity (Kruess and Tscharntke 2002). Over-grazing can remove enough flowering resources to decrease bumble bee populations (Carvell 2002; Hatfield and LeBuhn 2007) and destroy potential nest sites through trampling (Sugden 1985). Intensive grazing can also affect butterfly populations through trampling (Warren 1993) and altered plant community composition (Stoner and Joern 2004). Development of a grazing plan that includes careful consideration of the type of grazer, its food preference, and how well it can be managed is important for managing invasive species and compatibility with pollinators and other resources. For example, goats and sheep can be controlled through herding when they are brought onto a site and when they are removed, thereby bracketing periods of time when pollinators and pollinator habitat are least affected. Goats and sheep prefer broadleaf plants and therefore are the preferred grazers for sites where broadleaf weeds are an issue. At specific densities and durations, goats and sheep can control large infestations of invasive weeds, including weeds in areas near water, thereby reducing effects on water quality. Goats and sheep can also be effective in inaccessible spots, such as steep slopes. It is best to introduce grazers at a time when they preferentially feed on the target weed species. Grazing is most effective, for example, when target weeds are palatable; however, this period may be detrimental to pollinators. If rare or imperiled pollinators are present, timing grazing so as to avoid breeding and foraging periods is best. Avoiding grazing during the adult flight period or when imperiled butterfly or moth larvae are feeding on host plants reduces the effect on pollinators. It is also important to avoid grazing during periods when floral resources are already scarce because grazing during such times can eliminate pollinators from sites over time. Lastly, the stocking density of grazers can help to determine the duration of grazing. If stocking density is high, it is best to keep the duration relatively short so that desirable vegetation is not affected. Prescribed Fire Prescribed fire is used to manage roadside vegetation and rejuvenate plant diversity in some regions of the United States that have a history of natural fires. Prescribed fire can remove old vegetation to create room for new growth, can reduce the spread of some invasive plants, and can stimulate the seed germination of some wildflowers. Prescribed GoatsÂ canÂ beÂ usedÂ toÂ controlÂ noxiousÂ weedsÂ andÂ brushÂ onÂ someÂ roadsidesÂ orÂ otherÂ transportationÂ agencyÂ lands.Â PhotoÂ Credit:Â IdahoÂ TransportationÂ DepartmentÂ
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-27 fire can benefit pollinators through restoration or maintenance of suitable habitat (Huntzinger 2003), but it can have long-term effects on the populations of some species when refuges from burns are not available (Neâeman et al. 2000). Burns during the growing season destroy eggs, caterpillars, and aboveground nests and remove vegetation at a time when pollinators need floral resources, host plants, and nesting materials. Winter burns destroy species that overwinter in leaf litter or stems. Limiting the scale of the prescribed fire, making sure that some habitat remains unburned, helps to reduce impacts on pollinators. For example, an extremely large and expansive fire may kill pollinators overwintering in aboveground biomass. Such fires during the growing season may eliminate floral resources in a given area. Smaller dispersed fires, on the other hand, conserve floral resources and support pollinators in the area by providing refuge. Limiting the scale and frequency of each burn are important pollinator-friendly practices. It is best to use prescribed burning on sections of the roadside corridor rather than the entire corridor. By leaving unburned roadside habitat, enough pollinators remain to recolonize the burned areas. Rotational burning, such as burns conducted 3 to 5 years apart or more, allow time for pollinator populations to recover. Rotational burning can provide the benefits of prescribed fire without irreparably damaging the local pollinator community (Black et al. 2011). Varying the timing of prescribed burns can also reduce harm to pollinators by not continuously affecting certain pollinators and components of the roadside plant community. Burns affect pollinators no matter when the burn occurs. Therefore, altering the timing of burns can reduce negative effects on a particular group or suite of pollinators. It can be logistically challenging for DOTs to implement prescribed burns on roadsides. Staff or contractors require training and equipment to conduct burns, weather conditions must be amenable to burning (e.g., no high winds), warning signs and traffic control must be put in place to keep drivers safe, and other concerns must be addressed in order to implement burning safely and effectively. When these challenges can be met, prescribed burns can be a valuable component of roadside vegetation management. 6.3.3 Managing for Plant Diversity If a particular focal species of imperiled pollinator is not being considered during management, pollinator communities overall can be supported by managing roadside vegetation so as to maintain a diversity of plant species. There is a direct relationship between the diversity of blooming species and the diversity of pollinators; managing for plant diversity can provide a diverse community of pollinators. The following list provides guidance on the general timing of vegetation maintenance practices that best encourage plant diversity in this region. However, it is important to note that the timing can be in direct conflict with the activity of certain imperiled pollinator species. If the goal is to conserve a particular species (e.g., monarch butterflies), then carefully balance the timing of activities with the overall goals. ï· Mowing for diversity: AugâSept PrescribedÂ burnsÂ canÂ beÂ aÂ challengeÂ toÂ conductÂ onÂ roadsidesÂ dueÂ toÂ trafficÂ andÂ safetyÂ concerns,Â butÂ theyÂ canÂ beÂ beneficialÂ forÂ roadsideÂ vegetation.Â PhotoÂ Credit:Â IowaÂ LivingÂ RoadwayÂ TrustÂ FundÂ
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-28 ï· Grass-selective herbicides: Springâsummer ï· Haying: Not commonly used in this region ï· Prescribed fire: AugâOct ï· Prescribed grazing: Graze during peak growth of target weeds; grazing for diversity not common in this region 6.3.4 Road Salts and Heavy Metals Routine vehicle use and road maintenance contribute to roadside vegetation contamination by depositing pollutants, including vehicle exhaust and de-icing materials. Vehicle-derived heavy metal contamination in roadside soils and vegetation is proportional to traffic levels (Leharne et al. 1992; Mitchell et al. 2020) and generally highest adjacent to a road (Quarles et al. 1974; Dale and Freedman 1982; Jablonski et al. 1995; Swaileh et al. 2004). Heavy metal deposition can have harmful effects on some pollinators (e.g., Perugini et al. 2011); however, the concentrations that occur on roadsides may not reach levels that result in harmful effects (Mitchell et al. 2020). De-icing salts used on roads alter roadside soil chemistry by increasing sodium levels in soil and plant tissues significantly (Snell-Rood et al. 2014; Mitchell et al. 2020). Salts can damage some plants (Bogemans et al. 1989), which may indirectly affect pollinators but can directly affect butterfly and moth caterpillars that consume contaminated vegetation. Sodium is an important micronutrient for butterflies. Moderate levels can increase flight muscle and brain size in adults; however, too much can be toxic (Snell-Rood et al. 2014). Soil and leaf sodium levels are higher close to the road edge and on roads with more traffic; however, sodium levels reach lethal levels in only 1 percent of roadside plants in Minnesota (Mitchell et al. 2020). Concentrations of heavy metals or road salts in roadside soils and plants are not well documented across the United States. More data about the impacts of salts, heavy metals, and exhaust on pollinators are needed. Conservation recommendations for pollinators: ï· Maintain plant diversity in the roadside. Different plant species have different levels of heavy metal and salt uptake. By encouraging plant diversity, there will be variable concentrations of these toxins, which may help to reduce exposure to pollinator communities. ï· Increase the width of the mown strip in the recovery area along roadways where traffic is very high and in regions where road salts are frequently used. Until widespread data are available, DOTs that use road salts frequently in areas with high traffic levels may consider increasing the width of the mown strip in the recovery area. (Plants are more likely to be toxic when traffic levels exceed 20,000 cars per day.) Removing vegetation close to the road, an area where salt and heavy metal contamination is highest, will help reduce pollinator exposure to contaminants by removing the most toxic plants. This measure may not be necessary in all high-traffic or salt-heavy areas, but it is a step DOTs could take to reduce exposure for imperiled pollinators that use vegetation adjacent to the road.
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-29 6.3.5 Signage Signage can be used to help the maintenance staff identify areas that need particular management techniques or understand the timing of management that is unique to a particular site. The use of signs during maintenance may help the staff recognize sites with known populations of listed or imperiled pollinators, which may require specialized maintenance. For example, Oregon DOT has a Special Management Area (SMA) program, which was developed to formalize and organize the protection of rare and protected plants on Oregon roadsides. SMAs are established after roadside inventories of plants that are protected by state or federal legislation. More than 100 SMAs have been designated so far. SMAs also include compensatory wetland mitigation sites, cultural resources, and areas that need specialized maintenance. Signage designates each SMA site. The signs include codes for the particular maintenance activities (e.g., mowing, herbicide spraying, blading) allowed on the site. Each site has a unique management plan, which has been approved by the Oregon DOT Geo-Environmental Section, Oregon DOT district maintenance personnel, and relevant state and federal agency personnel. For additional information of the value of signage to the public, please see Chapter 11. 6.3.6 Beyond Roadsides If roadside maintenance practices cannot be adjusted in ways that support pollinators, there are other opportunities for pollinator habitat on land managed by DOTs that are not roadside rights-of-way. These areas include DOT âback 40â lots, stockpile lots, maintenance yards, underused DOT office land, special management areas, bicycle paths, scenic viewpoints, historic or geologic points of interest, stormwater ponds, or roadside rest areas. Such areas offer opportunities for collaboration with citizen groups because they are less exposed to traffic hazards, and DOTs may be more willing to allow volunteer groups to install or manage pollinator gardens in these areas. Â 6.4 CaseÂ StudiesÂ 6.4.1 Adjusting Mowing Practices to Benefit Monarch Butterflies in Illinois The monarch butterfly (Danaus plexippus) is the official state insect of Illinois. Spurred by low numbers of overwintering monarch butterflies in 2013 and 2014, the Illinois DOT began considering ways that mowing practices could be adjusted to support breeding and migrating monarchs in the state. Milkweed species, which serve as host plants for monarch caterpillars, can be abundant on Illinois roadsides, along with other wildflowers that serve as nectar sources for adult monarchs. In 2017, Illinois DOT altered mowing patterns to reduce the amount of roadside right-of-way that is mown regularly. By mowing only the 15 feet adjacent to the roadway, the goal was to leave more vegetation that could be used by monarchs. Mowing is still used when needed beyond the 15-foot zone to control the spread of invasive species and maintain sight lines for driver safety. Each year, Illinois DOT mows Milkweeds,Â hostÂ plantsÂ toÂ monarchÂ butterflies,Â canÂ beÂ abundantÂ onÂ roadsides.Â Â PhotoÂ Credit:Â Jennifer Hopwood/Xerces SocietyÂ SignsÂ canÂ provideÂ maintenanceÂ staffÂ withÂ directionsÂ ab utÂ anyÂ specialÂ m nagementÂ tha Â mightÂ beÂ ne dedÂ forÂ certainÂ sitesÂ that supportÂ imperiledÂ pollinators.Â Â PhotoÂ Credit:Â OregonÂ DOTÂ
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-30 one-third of the roadside, leaving the other two-thirds of roadside vegetation standing. Rotating mowing within different roadside strips every 3 years leaves a refuge from the mower that allows flowers to bloom, which supports pollinators, as well as produce seed and reproduce. Illinois DOT also times mowing to periods outside the peak breeding period of monarchs in Illinois. It also allows midsummer mowing, which can rejuvenate stands of common milkweed (Asclepias syriaca) and make those stands more attractive to monarchs that breed in late summer. For more information on how Illinois DOT is helping monarchs, visit: http://www.idot.illinois.gov/home/monarch and https://www.ilfb.org/media/5266/final_imp_mowingguidance_june2019_printversion.pdf. 6.4.2 Managing Plant Community Succession in Washington State The Washington State Department of Transportation (WSDOT) maintains about 100,000 acres of roadside property. WSDOT has been implementing an integrated vegetation management approach for many years, with the overarching goal of reducing undesirable vegetation while encouraging desirable vegetation. Many WSDOT practices and policies for managing vegetation also promote pollinator habitat. Whenever possible, WSDOT preserves existing native habitat, which can provide food, host plants, shelter, and nesting opportunities for pollinators. WSDOT also identifies roadsides for managed succession (WSDOT 2022). These are areas that have desirable vegetation that could flourish under a strategy that allows natural plant succession to proceed to a stable plant community. Outside the mown strip in the recovery area or sight lines where repeated mowing still occurs, roadsides managed for natural succession have multi-year treatment strategies that employ a variety of vegetation management tools, including reduced mowing and targeted herbicide use. Managed succession allows native vegetation to emerge and flourish, with sagebrush (Artemisia tridentata), greasewood (Sarcobatus vermiculatus), and native grasses taking hold in prairie regions and understory shrubs that are attractive to pollinators like snowberry (Symphoricarpos albus), Oregon grape (Mahonia nervosa), and spirea (Spiraea sp.) in other regions. When roadside revegetation is necessary, WSDOT prioritizes the use of native plants, including a diversity of native wildflowers and flowering shrubs and trees. Landscape designers focus on native plants that can establish with minimal input and compete with weeds. Key considerations for pollinators during the planning process include sequential bloom periods for flowering plants and high plant diversity. The maintenance staff is included in the planning process to provide input on long-lived plants that require minimal management. Mapping and planning are key elements of WSDOTâs approach to roadside revegetation and maintenance. Vegetation inventories help identify areas with weed infestations as well as areas that are conducive to managed succession. All maintenance personnel have tablets, which are used to record data and view maps and aerial images. NativeÂ trees,Â shrubs,Â wildflowers,Â andÂ grassesÂ allÂ thriveÂ onÂ thisÂ WashingtonÂ roadside.Â Â PhotoÂ Credit:Â Washington DOTÂ
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-31 WSDOT personnel use the best available science to inform their actions and undertake their own research to determine the right methods for their management needs. For example, experimental plots have been established to test seed mixes and planting methods. WSDOT also monitors the effectiveness of maintenance techniques on vegetation and pollinators. More information about the actions WSDOT is taking to protect pollinators and promote pollinator health can be found at: https://www.wsdot.wa.gov/NR/rdonlyres/767B37F7- EB70-4C0A-92FB-DD04F94965C6/0/PollinatorsFactSheet.pdf. 6.4.3 Maintenance Practices in Oregon Help Fenderâs Blue Butterfly Fenderâs blue butterfly (Icaricia icarioides fenderi), an endangered species that was once thought to be extinct, is known to occur only in the prairies of Oregonâs Willamette Valley. An estimated 99 percent of the valleyâs native prairies have been turned into farmland or otherwise developed. In addition to habitat loss, another major threat to the butterfly comes from invasive plants such as Himalayan blackberry and Scotch broom, which have outcompeted and displaced Kincaidâs lupine (Lupinus oreganus), the host plant for this species, as well as other native wildflowers in upland prairies where the butterflies live. Kincaidâs lupine is a threatened species. Both the plant and butterfly can be found on county-managed roadsides in the Willamette Valley. In Yamhill County, Kincaidâs lupine and Fenderâs blue butterfly occurred on both the foreslope of roadsides (i.e., from the edge of the roadway to the bottom of the roadside ditch) and the backslope of roadsides (i.e., the slope from the bottom of the ditch up to natural ground and the edge of the right-of-way). Routine maintenance activities like grading and herbicide spraying on the foreslope had negative impacts on Fenderâs blue butterfly and Kincaidâs lupine. Under these conditions, it became appropriate to reduce the attractiveness of the foreslope to Fenderâs blue butterfly by managing it so that Kincaidâs lupine would not persist. Working with USFWS, Yamhill County developed a habitat conservation plan for Fenderâs blue butterfly and Kincaidâs lupine. As part of the habitat conservation plan, Yamhill County conducted surveys of roadsides and designated and marked zones on the backslope of roadsides, which were designated Threatened and Endangered Special Maintenance Zones. Yamhill County maintenance personnel manage backslopes to minimize impacts on Kincaidâs lupine and Fenderâs blue butterfly. Yamhill County uses maps and Global Positioning System coordinates, as well as photos of Kindcaidâs lupine, for identifying and tracking of these sensitive areas. Yamhill County is also working to restore upland prairie at a mitigation site to provide habitat for these protected species. Yamhill Countyâs Habitat Conservation Plan is found here: https://www.fws.gov/oregonfwo/documents/hcp/YamhillHCP_Final.pdf. TheÂ endangeredÂ FenderâsÂ blueÂ butterflyÂ andÂ itsÂ hostplant,Â theÂ threatedÂ plantÂ KincaidâsÂ lupine,Â areÂ foundÂ onÂ someÂ ofÂ OregonâsÂ roadsides.Â PhotoÂ Credit:Â Candace FallonÂ
ChapterÂ 6.Â RoadsideÂ MaintenanceÂ andÂ VegetationÂ ManagementÂ forÂ Pollinators 6-32 6.5 AdditionalÂ ResourcesÂ Federal Highway Administration Environmental Review Toolkit for Pollinators: https://www.environment.fhwa.dot.gov/env_topics/ecosystems/pollinators.aspx.Â Literature Review: Pollinator Habitat Enhancement and Best Management Practices in Highway Rights-of-Way: https://www.environment.fhwa.dot.gov/env_topics/ecosystems/pollinator_reports/pollinators_BMPs_in_h ighway_ROW.pdf.Â Roadside Best Management Practices that Benefit Pollinators: Handbook for Supporting Pollinators through Roadside Maintenance and Landscape Design: https://www.environment.fhwa.dot.gov/env_topics/ecosystems/Pollinators_Roadsides/BMPs_pollinators_l andscapes.pdf.Â Pollinators and Roadsides: Best Management Practices for Managers and Decision-Makers: https://www.environment.fhwa.dot.gov/env_topics/ecosystems/Pollinators_Roadsides/BMPs_pollinators_ roadsides.pdf.Â Identifying the Current State of Practice for Vegetation Management Associated with Pollinator Health and Habitat: An Interview Report: https://www.environment.fhwa.dot.gov/env_topics/ecosystems/pollinator_reports/pollinator_interview_r pt.aspx.Â Mowing and Management: Best Practices for Monarchs: https://monarchjointventure.org/images/uploads/documents/MowingForMonarchs.pdf.Â Monarch Butterflies, Weeds, and Herbicides Fact Sheet: https://monarchjointventure.org/images/uploads/documents/HerbicideFactsheet.pdf.Â Evaluating the Suitability of Roadway Corridors for Use by Monarch Butterflies: https://www.nap.edu/catalog/25693/evaluatingâtheâsuitabilityâofâroadwayâcorridorsâforâuseâbyâmonarchâ butterfliesÂ and http://www.trb.org/Main/Blurbs/180186.aspx.Â Milkweed Recognition Guides: https://www.xerces.org/monarchs/roadsides.Â Â